1. Field of the Invention
The present invention relates to a multi-beam exposure method and an apparatus thereto in which exposure is performed with a predetermined pattern by focusing a light beam for each single pixel with an optical element to irradiate a recording medium, the light beam being emitted from a spatial light modulation element (two-dimensional light modulator) or the like which is a means for selectively turning on and off the plural pixels arranged in an exposure head in accordance with the image data (pattern data).
2. Description of the Related Art
Recently, multi-beam exposure apparatuses are being developed, in which a spatial light modulation element (two-dimensional light modulator) such as a digital micromirror device (DMD) is used as a pattern generator to perform image exposure on a recording medium such as a planographic printing plate with the light beam modulated according to image data. Because the spatial light modulation element (two-dimensional light modulator) has a configuration in which the pixels for modulating incident light are two-dimensionally arrayed, the spatial light modulation element has an advantage that a spatially incoherent light source such as a lamp can also be used with the spatial light modulation element.
Depending on the method of using the spatial light modulation element (two-dimensional light modulator), light power per unit pixel can be reduced in the spatial light modulation element when compared with a one-dimensional light modulator in which the pixels are one-dimensionally arrayed, and the life of the spatial light modulation element can be lengthened. Therefore, it is expected that such a spatial light modulation element will be widely used for multi-beam exposure apparatuses.
A DMD is a mirror device in which many micromirrors are two-dimensionally arrayed on a semiconductor substrate such as silicon. In the micromirror, for example, an angle of a reflection plane is changed according to a control signal. The micromirror is configured to change the angle of the reflection plane using the electrostatic force of an electrical charge accumulated in each memory cell.
In a multi-beam exposure apparatus in which a DMD is used, a laser beam emitted from a light source is collimated by a lens system and reflected by the plural micromirrors in the DMD arranged at substantially the focal position of the lens system. Therefore, the light beams two-dimensionally modulated and emitted are focused for each single pixel by the lens system to form a spot having a small diameter on an exposure surface of a recording medium (planographic printing plate) which is a photosensitive material, effecting imagewise exposure.
Namely, in this multi-beam exposure apparatus, each of the micromirrors in the DMD is on/off-controlled by a control device (not shown) based on the control signal generated according to the image data or the like, the laser beam is modulated (deflected), and the exposure is performed by irradiating the exposure surface with the modulated laser beam.
In this multi-beam exposure apparatus, the planographic printing plate or the like is placed on a recording surface, and the DMD is modulated according to the image data while the position of the beam spot is moved relative to the planographic printing plate, with the image of the beam spot being formed on the planographic printing plate by irradiating the planographic printing plate with the laser beams emitted from the plural exposure heads provided in the exposure apparatus. Therefore, the multi-beam exposure apparatus can perform a process of exposing a pattern on the planographic printing plate. Further, the recording medium such as the planographic printing plate into which the image has been recorded is subject to automatic development by an automatic developing machine if necessary, and a latent image formed in the recording medium such as the planographic printing plate is converted into a visual image.
Conventionally, in DMD used for such multi-beam exposure apparatuses, the micromirrors are arrayed in m rows in a scanning direction and n columns in a direction orthogonal to the scanning direction. Further, in such a DMD, the rows of the pixels are arranged so as to be inclined at a predetermined angle relative to the scanning direction of the exposure head, so that (m/N−1) dots can be formed between scanning lines by performing multiple exposures N-times at staggered time intervals in the scanning direction.
Thus, it is proposed that a dot pitch is changed by adjusting the number of multiple-exposure times in the scanning direction, to increase addressability in the direction orthogonal to the scanning direction (for example, Japanese Patent Laid-Open Publication No. 2004-62156).
However, in such a multi-beam exposure apparatus, when the rows of the pixels of the DMD are arranged so as to be inclined at the predetermined angle relative to the scanning direction of the exposure head to perform recording, by a multiple-exposure method at staggered time intervals in the scanning direction, one dot is formed on the planographic printing plate by performing multiple exposures N-times at staggered time intervals for pixels which are inclined relative to a main scanning line. At this point, in the planographic printing plate, a position of each beam spot in which the multiple exposures are performed in order to record one dot is shifted in a sub-scanning direction. Therefore, an exposure amount distribution in the recorded dot extends in the sub-scanning direction, and the recorded dot is formed as a blurred image in the sub-scanning direction.
In the conventional multi-beam exposure apparatus in which the image exposure is performed onto the planographic printing plate, when the image exposure is performed onto the planographic printing plate to express a halftone, usually the halftone is expressed with a dot shape (a so-called micro-checkered pattern) by utilizing an AM screen (a technique of forming a gray-scale image by a dot image). Namely, in the AM screen, the minimum-unit of halftone dot image is formed by a relatively large number of dots such as 196 dots of 14 (the number of dots in the horizontal direction) by 14 (the number of dots in the vertical direction), and the gray-scale image is recorded by recording the dot images while arranging the dot images in a two-dimensional plane. However, when the halftone is expressed by utilizing the AM screen, sometimes a moiré pattern or a tone jump is generated.
As a method to form a gradation image using a dot image, there is an FM screen technique of forming the gray-scale image by dot images. An FM screen expresses the gray scale of the recording image by the cumulative density of irregular patterned-dots having no regularity. For example, in an FM screen, the images formed by a relatively small number of dots such as a total of four dots of two by two are dispersed in the two-dimensional plane to perform the gray-scale expression. An FM screen has the advantage that the generation of a moiré pattern or the like can, in principle, be suppressed.
Therefore, in a conventional multi-beam exposure apparatus in which the image exposure is performed onto a recording medium such as a planographic printing plate, it is desired that the halftone be formed in a small dot shape using an FM screen.
However, in a multi-beam exposure apparatus, when the rows of pixels of the DMD are arranged so as to be inclined at a predetermined angle relative to the scanning direction of the exposure head to perform recording by the N-times multiple-exposure method at staggered time intervals in the scanning direction when performing the multiple exposures in order to record one dot on the recording medium such as the planographic printing plate, the position of the plural beam spots in which the multiple exposures are performed are shifted in the sub-scanning direction relative to the same exposure point which should be formed into one dot, and the exposure amount distribution in the recorded dot extends in the sub-scanning direction. Therefore, sometimes the recorded dot is formed as a blurred image in the sub-scanning direction.
In an image in which the exposures are performed to record the FM screen, when each dot is thus recorded as a blurred shape in the sub-scanning direction, that is, under conditions in which the edge cannot be kept sharp, even a slight change in the circumferential length of the recording pixel generated depending on the recording conditions such as a light power fluctuation and the number of prints or development conditions such as a degree of development of an automatic developing machine could cause a rapid change in a dot-image ratio (dot-coverage ratio characteristic), resulting in a change in image density. Therefore, there is a problem that an FM screen is difficult to use in conventional multi-beam exposure apparatuses which adopt the method in which the rows of the pixels of a DMD are arranged so as to be inclined at a predetermined angle relative to the scanning direction of the exposure head to perform recording by the N-times multiple-exposure method at staggered time intervals in the scanning direction.